The automatic nervous system (ANS) regulates “involuntary” organs and maintains normal internal function and works with the somatic nervous system. The ANS includes the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system is affiliated with stress and the “fight or flight response” to emergencies, and the parasympathetic nervous system is affiliated with relaxation and the “rest and digest response.” Autonomic balance reflects the relationship between parasympathetic and sympathetic activity. Changes in autonomic balance are reflected in changes in heart rate, heart rhythm, contractility, remodeling, inflammation and blood pressure. Changes in autonomic balance can also be seen in other physiological changes, such as changes in abdominal pain, appetite, stamina, emotions, personality, muscle tone, sleep, and allergies, for example.
It is desirable to use a measurement of autonomic balance in order to appropriately control or titrate various neural stimulation therapies. Neural stimulators have been proposed to treat a variety of disorders, such as epilepsy, obesity, breathing disorders, hypertension, post myocardial infarction (MI) remodeling and heart failure. Direct electrical stimulation has been applied to the carotid sinus and vagus nerve. Research has indicated that electrical stimulation of the carotid sinus nerve can result in reduction of experimental hypertension, and that direct electrical stimulation to the pressoreceptive regions of the carotid sinus itself brings about reflex reduction in experimental hypertension. Electrical systems have been proposed to treat hypertension in patients who do not otherwise respond to therapy involving lifestyle changes and hypertension drugs, and possibly to reduce drug dependency for other patients. The stimulation of sympathetic afferents triggers sympathetic activation, parasympathetic inhibition, vasoconstriction, and tachycardia. In contrast, parasympathetic activation results in bradycardia, vasodilation and inhibition of vasopressin release. Direct stimulation of the vagal parasympathetic fibers has been shown to reduce heart rate. In addition, some research indicates that chronic stimulation of the vagus nerve may be of protective myocardial benefit following cardiac ischemic insult. Reduced autonomic balance (increase in sympathetic and decrease in parasympathetic cardiac tone) during heart failure has been shown to be associated with left ventricular dysfunction and increased mortality. Research also indicates that increasing parasympathetic tone and reducing sympathetic tone may protect the myocardium from further remodeling and predisposition to fatal arrhythmias following myocardial infarction.